I am attempting to build a CB radio (26-27 MHz) receiver using the USRP
with the LFRX daughterboard. Currently, a “rubber-ducky” CB antenna is
connected directly to the LFRX and the DDC on the USRP converts the
received signal to baseband.

At the receiver output, I am seeing a roughly 250Hz sinusoid. I am not
sure where this is coming from; if I attach a scope_sink directly to
usrp.source_c(), I still see the unwanted signal. The signal remains no
matter what CB channel I listen to.

I also tried using the BASIC Rx with a 30MHz lowpass filter, with the
same results.

Any ideas as to what I can do to eliminate this noise? My code is below.

return self.set_freq(channels[int(channel - 1)])
def set_freq(self, target_freq):
"""
Set the center frequency we're interested in.
@param target_freq: frequency in Hz
@rypte: bool
Tuning is a two step process. First we ask the front-end to
tune as close to the desired frequency as it can. Then we use
the result of that operation and our target_frequency to
determine the value for the digital down converter.
"""
r = usrp.tune(self.u, 0, self.subdev, target_freq)
if r:
self.freq = target_freq
self.update_status_bar()
self._set_status_msg("OK", 0)
print "r.baseband_freq =",